1. Field of the Invention
The present invention relates to sidelight-type surface lighting devices which use a light guide member having a side surface for reception of light from the light source. The present invention also relates to liquid crystal display devices which use such a surface lighting device serving as lighting means in the form of a backlight unit for example.
2. Background Art
Liquid crystal display devices have been finding expanding fields of application due to advantages including low power consumption, light-weight and thinness. Liquid crystal display devices are now commonly used as residential/commercial image monitors, industrial monitoring devices, and display devices for mobile communications terminals. These liquid crystal display devices are often provided with a backlight unit or a surface lighting device since liquid crystals do not illuminate by themselves and providing light from behind the liquid crystal panel is often preferred.
Most of the backlight units used in the liquid crystal display devices are sidelight type so that the whole device can be as thin as possible. Typically, the sidelight-type backlight includes a tubular light source called edge light, which is a cold cathode tube placed on a side surface of a light guide plate. Light from the edge light enters the light guide plate from the side surface, and then comes out from the upper surface or main surface, illuminating the back surface of the liquid crystal panel. Thus, the backlight unit serves as a surface lighting device which provides lighting from behind the liquid crystal panel.
In response to increasing requirement for reduction in the number of parts, decrease in thickness and increase in brightness, use of prism light guide plates is increasing in liquid crystal display devices for large-sized, notebook-type personal computers.
FIG. 18 is a sectional view of a conventional liquid crystal display module (hereinafter called Conventional Example 1) used in notebook PCs (notebook-type personal computers). This liquid crystal display module 200 includes a metal bezel 202 formed typically by press. The bezel 202 serves as a frame, to which a resin chassis 207 is fitted as a supporting member. A main body of the liquid crystal display module 200 includes two components: a liquid crystal panel 211 which includes a liquid crystal layer and two glass substrates sandwiching the liquid crystal layer; and a backlight unit 212 serving as a surface lighting device. The main body is supported or held by the chassis 207 and housed in the frame. The backlight unit 212 includes a prism light guide plate 203 supported by the chassis 207, an optical sheet 205 and a protective sheet 206 placed one after the other on an upper surface of the prism light guide plate 203, and a reflective plate 204 placed on a lower surface of the prism light guide plate 203. The backlight unit 212 further includes: an edge light (hereinafter simply “lamp”) 208, which is a single-tube, cold-cathode tube placed closely to a side surface of the prism light guide plate 203; a stainless-steel lamp reflector 209 which reflects the light from the lamp 208; and lamp holders 210 which hold the lamp 208. The lamp reflector 209 covers all side surfaces of the lamp 208 except one facing the prism light guide plate 203, allowing light from the lamp 208 to enter the side surface of the prism light guide plate 203. The light that enters from this side surface (hereinafter called “light incident surface”) serves as illumination light, i.e. comes out of an upper surface or the main surface of the prism light guide plate 203, and thereby provides lighting from behind the liquid crystal panel 211.
FIG. 19 is a plan view of a backlight unit in another conventional liquid crystal display module (hereinafter called Conventional Example 2: See the U.S. Pat. No. 6,512,557 which corresponds to the Japanese Patent Laid-Open 2001-108988.) In this Conventional Example, a light guide plate 303 includes projections 303a, 303b which are fitted into a frame 307, thereby fixing and positioning the light guide plate 303 with respect to the frame 307.
However, such projections as the above cannot be provided on the light guide plate if the backlight unit uses a prism light guide plate as in Conventional Example 1, in view of maintaining uniform brightness required of the surface lighting device. With the limitation that the fixing means based on such projections cannot be used, there must be some other means for securing the light guide plate. In an attempt to solve this problem, there has been made a proposal that a position-shift prevention mechanism be provided on the light incident surface of the light guide plate, i.e. on the surface facing the edge light or the cathode tube. The proposal is applicable if the lamp reflector is made of PET (polyethylene terephthalate) resin or other elastic materials. However, if the material is not elastic as in Conventional Example 1 which uses a stainless-steel lamp reflector 209, it becomes difficult to assemble the lamp reflector into place. As a result, it becomes difficult to provide the stopping means on the lamp housing (frame).
Thus, in the case represented by Conventional Example 1 where the backlight unit uses a prism light guide plate and a rigid lamp reflector made of e.g. stainless steel for improved efficiency in the use of light, it is impossible to realize a positioning mechanism by providing stopping means on the lamp housing (frame). Under this limitation, the lamp holders 210 are supposed to serve as a positioning mechanism for the light guide plate, and this poses a problem of reduced impact resistance especially in large-sized liquid crystal display modules.
In an attempt to solve this, the Japanese Patent Laid-Open 2000-331521 discloses a sidelight-type surface lighting device (and liquid crystal display devices using the same), in which a light guide plate includes a light entering surface (light incident surface) having two ends each provided with a position-shift prevention mechanism. The mechanisms are located where the light incident surface is faced by electrodes of a stick-like light source serving as an edge light.
According to this surface lighting device, the position-shift preventing mechanism is provided by a projection projected from an inner wall surface of a frame that holds the light guide plate, or as part of a metal reinforcing member for the frame that holds the light guide plate.
A problem here is that it is difficult to give these projections a sufficient thickness when the position-shift prevention mechanism is to be a projection from a resin frame, in view of making the frame width as narrow as possible. Further, there is a problem related to assembling, i.e. it is almost impossible to attach a lamp and a lamp reflector to a resin frame.
On the other hand, if the position-shift prevention mechanism is to be part of the reinforcing metal member, there is a problem of impact resistance, because the mechanism will usually be formed by cutting and removing a portion of the metal member. The cutting sacrifices strength since the metal material generally used for the lamp reflector is only as thick as 0.188 mm. If the metal thickness is increased, the resultant thickness of the module will have to be 6.5 mm or over, which will reduce merchantability of the liquid crystal display devices (liquid crystal display module). It should be appreciated that a lamp reflector surrounds the lamp except a part close to the light incident surface of the light guide plate. For this reason, a 0.5 mm addition to the thickness of lamp reflector metal plate will cause a total thickness addition of: 0.5 mm×2=1.0 mm.
Further, if the reinforcing metal member has a cutout, there is no but one way of assembling the lamp into the lamp reflector, i.e. by inserting the lamp longitudinally thereof, into the lamp reflector. This must be done carefully enough so that the lamp is not damaged, leading to reduced operability in the assembling process. For the same reason, it is not easy to replace the lamp, either.
Thus, according to the disclosure in the Japanese Patent Laid-Open 2000-331521, the position-shift prevention mechanism will not have a high strength, or the reinforcing metal member will have a reduced strength. Therefore, it is impossible to achieve sufficient impact resistance without sacrificing a desired narrowness in the frame of the display device.